Wood is awesome. We love the grain, texture, and even the smell of engraved wood. Laser engraving wood might seem pretty simple at first, but in reality, it's tough to get a sharp, dark, and clean engraving.
When you use a laser engraver, the laser beam burns the wood and vaporizes the material. When wood is vaporized through burning, it creates smoke. As the smoke exits the laser bed through the exhaust, it is drawn across the surface of the wood. This smoke will often leave a sticky yellow or brown stain on the wood.
One technique we use is in our workshop in Maryland, is to cover the surface of the wood with low tack masking tape before engraving. We buy really wide rolls of this tape, allowing us to quickly cover surface of the wood that we're engraving. Then, we run the laser, and after the job is complete, peel off the tape. The tape protects the wood surface from burn marks, making for a much easier clean up. Since it's low-tack tape, it's pretty easy to take off as well. Keep in mind, if you are doing a very detailed engraving, it can be a little time consuming to take the tape off after engraving, so we use a piece of acrylic or a flexible paint scraper to speed things up.
If you want a very sharp engraving and a nice dark contrast for your wood, consider increasing your engraving resolution. The more dots per inch (DPI) you use, the more times the laser will fire, burning the wood darker. Increasing your DPI will make the job take twice as long, so if you are going for fast production, you probably don't want to go this route.
Set your laser to manually focus. After you've set it to the appropriate focus depth, use a digital caliper and drop your laser bed .2". We've found that this still keeps the engraving pretty sharp, but for whatever reason produces much less smoke and creosote. This saves a ton of time in cleanup.
For every wood engraving, we always clean the surface of the wood after engraving. By filling a squirt bottle with denatured alcohol, you can quickly spray some on the surface of the wood, and clean off burn marks. We use white cotton paint rags, and work the surface in a circular motion. Be sure not to use a colored rag, or the dye could stain your wood surface.
When we first started using our laser, a friend recommended we use cabinet-grade plywood for wood engraving. Unbeknownst to us, plywood construction is actually pretty complicated. You have different core types (MDF or veneer core), different veneer grades for the face, and different glue types. Certain glue types interfere with how a laser cuts, which can make a thin sheet of plywood very difficult to cut. Also, with veneer core plywood, you often will get garbage veneers for the core, even if the outside veneer is flawless. Many times the core veneer will have a void in it, leaving an ugly gap in your cut edge.
Also, plywood doesn't really engrave that nicely. You have to really dial in your laser to get a good dark burn. And if you burn through the top layer of veneer, you are going to expose the first sheet of the veneer core which will have the wood grain running in the opposite direction. That can look pretty crappy. Combine that with poor grade veneer cores, and you are going to run into all kinds of headaches.
We decided to start using solid wood panels for a few reasons. First, they just look better. Nothing beats having the engraving expose more of the grain, running in a consistent direction. Second, hardwood has more weight to it than plywood, which means it'll feel like it's of a higher quality. Lastly, you can have absolute control over the material surface. We worked really hard to find partners to supply our glue ups, with very exacting specifications. Our glue ups are always made from the same piece of wood, with as wide boards as possible, and with no knots or defects.
We look at our laser as another tool at our disposal. By focusing on experimentation, craftsmanship, and attention to detail, you'll constantly be challenging yourself to come up with better and better work.